This invention relates to a process for the removal of hydrogen sulfide from gases by passing the gases in the presence of oxygen and steam over a catalytic absorption mass which contains inert porous support materials as support and catalytically active metal sulfides and/or metal oxides for the selective oxidation of hydrogen sulfide to elemental sulfur, the sulfur formed being simultaneously deposited on the absorption mass, and regenerating the charged catalytic absorption mass.
The removal of hydrogen sulfide from industrial gases, for example coal gas, has long been a problem. Before 1940, hydrogen sulfide was removed from coal gas by reaction with moist iron oxide at temperatures below about 100.degree. C. When the uptake of sulfur had reduced the reactivity of the oxide, the absorption mass was regenerated by treatment with air. Through the reaction with oxygen, the iron sulfide formed was slowly converted into iron oxide and elemental sulfur.
In order to obtain a relatively high uptake of sulfur, a little oxygen was added to the coal gas. The iron oxide then acted as a catalyst which produced a reaction between the hydrogen sulfide and sulfur at low temperatures. The elemental sulfur formed remained in the iron oxide and reduced the catalytic activity. Because it was difficult to extract the sulfur deposited from the catalyst mass, the deactivated catalyst was generally discarded. The frequent renewal of the catalyst was of course a technical disadvantage.
On account of these disadvantages, the removal of hydrogen sulfide was carried out continuously by absorption in liquid phase. Hydrogen sulfide is absorbed in organic liquids, such as methanol or alkanolamines. Although this process has advantages over the process using iron oxide, it also has certain disadvantages.
A low-temperature desulfurization process is described in European patent application 98 444. In this process, a fluid absorption mass based on iron hydroxides contained in absorption towers is passed in countercurrent to the gas containing the hydrogen sulfide. However, the charged absorption mass loses it catalytic activity because the sulfur is deposited on the surface and the catalytic absorption mass cannot be regenerated in this process.
A similar low-temperature gas desulfurization process is described in The Chemical Engineer, November 1984, page 30. This process is used to remove hydrogen sulfide from natural gas from North Sea oil sources. In this process, hydrogen sulfide is removed by reaction with granular zinc oxide in fixed-bed reactors. However, the disadvantage of this process lies in the fact that the absorbing material cannot be regenerated and the fixed-bed reactors have a limited life before they have to be renewed.
In addition to dry processes for the removal of hydrogen sulfide, various processes have been developed for the wet-chemical oxidative removal of hydrogen sulfide. In the Linde Sulfolin process, which is described in Wissenschaft und Technik, Vol. 8, page 371, 1986, hydrogen sulfide is removed from the gas to be purified by absorption in an aqueous soda solution and is oxidized to sulfur by means of sodium vanadate.
In the LO-CAT process, which is described in Chemical Engineering, page 62, 1985, the hydrogen sulfide passed into an alkaline aqueous solution of Fe.sup.3+ ions is directly oxidized to elemental sulfur. However, this wet-chemical process is attended by the disadvantages that the absorption solutions have only a very limited uptake capacity and have to be discarded after the process. Further disadvantages of this process include the expensive plant required, the high investment costs and the complicated operation of the plants.
An improved process for the removal of hydrogen sulfide is described in Hydrocarbon Processing, 1986, page 37. In this process, reaction vessels containing zinc oxide are used to absorb hydrogen sulfide and are regenerated by heating in an oxygen-containing air stream. Although regeneration of the absorption mass containing zinc oxide enables it to be reused, the sulfur dioxide formed in the process presents further disposal problems. Another disadvantage of this process is that, after several absorption cycles, the absorbable surface is reduced to 10% of the original surface.
DE-OS 21 44 567 describes the desulfurization of hydrocarbons with a desulfurization mass consisting of a porous support having a large specific surface to which divalent copper oxide is applied. This mass is produced by impregnation of the porous support material with an aqueous copper salt solution and drying. The mass obtained, even after calcination, contains the copper oxide in coarse granules, as is also the case with the other known reaction masses described above. Accordingly, desulfurization with this mass can only be carried out at relatively low temperatures because the mass sinters at relatively high temperatures. Regeneration is complicated and involves several process steps. The quantity of sulfur absorbed decreases greatly with increasing number of regeneration cycles (cf. Example 5 of DE-OS 21 44 567).
U.S. Pat. No. 4,478,800 (corresponding to EP-A-0 071 983) describes a process for the removal of hydrogen sulfide, carbonyl sulfide and/or carbon disulfide from gases which comprises the steps of:
a) passing the gas containing hydrogen sulfide, carbonyl sulfide or carbon disulfide at a temperature of 5.degree. to 800.degree. C. over an absorption mass, including metal oxides, which react with the sulfur compound, metal-sulfur compounds being obtained and these metal oxides being present on an inert, refractory support material having a specific surface of more than 10 m.sup.2 per g, PA1 b) charging the support material with the metal oxide in a quantity of at least 5% by weight, expressed as metal of the active component and based on the weight of the support material, PA1 c) at least 20% of the metal oxides on the support material being present in finely divided form with a particle size of less than 40 nm and PA1 d) regenerating the support material charged with metal-sulfur compounds by oxidation of these metal-sulfur compounds by passing over gases, including oxidizing agents.
This process gives excellent results. However, the quantity of sulfur with which the absorption mass can be charged is relatively low compared with the results of the present invention. Although, according to U.S. Pat. No. 4,478,800, the removal of hydrogen sulfide and other sulfur compounds can be carried out at a temperature of 5.degree. to 800.degree. C., temperatures above 300.degree. C. only are used in the Examples. In addition, the gases are preferably reducing gases, reducing gases being used in all the Examples.
A key feature of U.S. Pat. No. 4,478,800 is that the regeneration of the charged absorption mass is carried out by oxidation. Surprisingly, it was possible by regeneration of the absorbents in accordance with U.S. Pat. No. 4,478,800 directly to obtain elemental sulfur providing certain, active finely divided metal oxides, such as iron oxide, are used in the absorption mass.
Since there is a growing demand for improved processes for the removal of hydrogen sulfide from gases, it is an object of the present invention to provide a process which can be carried out at low temperatures and even at room temperature, hydrogen sulfide being removed substantially quantitatively from the gases, the same mass being capable of repeated regeneration and reuse for the removal of H.sub.2 S and elemental sulfur being directly obtained with no formation of sulfur oxides.